Organic Chemistry

(Dana P.) #1
Section 29.3 Electrocyclic Reactions 113

Table 29.1 Woodward–Hoffmann Rules for Electrocyclic Reactions

Number of Allowed mode
conjugated bonds Reaction conditions of ring closure

Even number Thermal Conrotatory
Photochemical Disrotatory
Odd number Thermal Disrotatory
Photochemical Conrotatory

p

“allowed by orbital symmetry.”It can be rather burdensome to memorize these rules
(and worrisome if they are forgotten during an exam), but all the rules can be summa-
rized by the mnemonic “TE-AC.”How to use “TE-AC”is explained in Section 29.7.


PROBLEM 5

a. For conjugated systems with two, three, four, five, six, and seven conjugated bonds,
construct quick molecular orbitals (just draw the porbitals at the ends of the conjugat-
ed system as they are drawn on p. 000) to show whether the HOMO is symmetric or
asymmetric).
b. Using these drawings, convince yourself that the Woodward–Hoffmann rules in
Table 29.1 are valid.
c. Using these drawings, convince yourself that the “TE-AC”shortcut method for learning
the information in Table 29.1 is valid (see Section 29.7).

PROBLEM 6

a. Under thermal conditions, will ring closure of (2E, 4 Z, 6 Z, 8 E)-decatetraene be conrota-
tory or disrotatory?
b. Will the product have the cis or the trans configuration?
c. Under photochemical conditions, will ring closure be conrotatory or disrotatory?
d. Will the product have the cis or the trans configuration?

The series of reactions in Figure 29.4 illustrates just how easy it is to determine the
mode of ring closure and therefore the product of an electrocyclic reaction. The
reactant of the first reaction has three conjugated double bonds and is undergoing ring
closure under thermal conditions. Ring closure, therefore, is disrotatory (Table 29.1).
Disrotatory ring closure of this reactant causes the hydrogens to be cis in the ring-
closed product. To determine the relative positions of the hydrogens, draw them in the
reactant and then draw arrows showing disrotatory ring closure (Figure 29.4a).


p

X-Ref

100 °C − 20 °C 25 °C

ab c

H

H

H

H H

H

HH

HH

h

disrotatory
ring closure

conrotatory
ring closure

disrotatory
ring closure

>Figure 29.4
Determining the stereochemistry of
the product of an electrocyclic
reaction.
Free download pdf